This article analyzes the invalidity of contracts in the context of private enforcement of competition law. Starting with an exam of the goals of competition law, this article argues that there are situations where the invalidity of contracts that substantiate a violation of competition law is not always the optimal way of enforcing competition law. In some circumstances, such as in the case of vertical agreements, there is however little legal room not to declare any such contract invalid. In regards to abuse of dominant position, a possible interpretation of article 294.º of the Portuguese Civil Code is proposed as to allow that some contracts that operate an abuse of dominant position not to be declared invalid when such sanction proves to be counter-productive.